ART Pro Audio TPS II - Tube Preamplifier System - Manual (Page 13)

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If you experience a grounding hum when using both output connectors
(one to a console, one to an instrument amp) simultaneously, a ground
loop may be the problem. To remedy this problem, disconnect the
ground wire (pin 1) from the XLR cable plugged into the preamp’s
output. This interrupts the ground path, and therefore breaks the loop.
TPS II™ signal Flow Diagram
The above diagram outlines the signal flow of the TPS II™. It provides
for a handy optical reference for what is occurring to the signal, and the
various features of the TPS II™, and their location and interaction with
the signal.